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Lopez Ortega, Alejandro

Publications and source records attributed to Lopez Ortega, Alejandro.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Dynamic thermal behavior of polycrystalline LaB 6 hollow cathodes

Lanthanum hexaboride (LaB6) hollow cathodes have demonstrated a capability for long life operation, which is critical to many space exploration missions. Thermal characterization of LaB6 hollow cathodes has revealed lower than expected electron emitter temperatures when the cathode reaches a steady state. This phenomenon is observed at discharge currents ranging from 5 to 35 A and xenon mass flow rates of 5–25 SCCM in cathodes with three different orifice diameters. Thus, the currently accepted value of the work function for polycrystalline LaB 6 , 2.67 eV, does not describe well the emission characteristics of LaB6 hollow cathodes operating with internal gas discharges at a steady state. We use empirically measured temperatures combined with a model of the hollow cathode emitter and xenon discharge to estimate the value of the work function, yielding a value ranging from 2.1 to 2.44 eV. This lower work function value implies that LaB6 hollow cathodes are expected to have even longer lifetimes than previously anticipated, further establishing them as a more suited alternative to other conventional cathode technologies for the task of long duration travel. Direct measurements of the work function as a function of depth on a hollow cathode emitter using x-ray photoelectron spectroscopy and ion beam milling indicate that the work function decreases with depth. We postulate several mechanisms that could explain the observed work function enhancement. Altogether, our results have important implications to the design, study approach, and operation of LaB 6 cathodes and potentially other cathodes with hollow configuration. Finally, our work opens the question of why the work function is reduced upon interaction with Xe plasma.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2021

Self-Consistent Implementation of a Zero-Equation Transport Model Into a Predictive Model for a Hall Effect Thruster

The performance of an axisymmetric multi-fluid Hall thruster code that incorporates a self-consistent, data-driven closure model for the anomalous electron transport is investigated. Five different operating conditions of the H9 magnetically shielded Hall thruster are simulated with the Jet Propulsion Laboratory’s Hall2De. In order to capture the inherent uncertainty associated with the closure model, O(100) simulations are run for each condition, each of using a coefficient set sampled randomly from a probability distribution. The results of these simulations provide probabilistic predictions of thruster performance quantities including thrust, and discharge current, as well as several component efficiencies and centerline plasma properties. The model is found to yield converged solutions at all conditions, with large 10 kHzrange oscillations and performance trends with voltage and flow rate similar to experiment. The model under-predicts the thrust by 15-25% and over-predicts the discharge current by 20% on average compared to experiments at the same discharge voltage and mass flow rate. This performance discrepancy is due to lower beam utilization, mass utilization, and divergence efficiency than experiment, resulting from high Hall parameters in the acceleration region, which lead to a protracted ion acceleration region. The physical processes underlying this result are discussed in the context of future data-driven modeling efforts.

Jorns, Benjamin A.↗
NASA NTRS2020

Anomalous Ion Heating and Pole Erosion in the 12.5-kW Hall Effect Rocket with Magnetic Shielding (HERMeS)

Wear measurements of the inner pole cover in HERMeS have shown as much as ten times higher erosion rates than past numerical simulations predicted. Also, measurements of ion velocity distribution functions (IVDF) suggest that ion heating occurs near the chamfered regions of the acceleration channel and around the inner pole surface. In our companion paper we present theoretical evidence that the plasma conditions in this region can excite instabilities in the lower hybrid frequency. Here, we provide further evidence of their presence through comparisons between numerical simulations and measurements. After including in the 2-D axisymmetric hydrodynamic-PIC code Hall2De an idealized model of the ion temperature based on a scaling with the lower hybrid frequency, we find the agreement between the computed and measured IVDF is significantly improved. In addition, the computed erosion rates at the inner pole surface are now in close agreement with wear test measurements.

Frieman, Jason↗
NASA NTRS2020

Hall2De Simulations of a Magnetic Nozzle

The two-dimensional, fluid-based Hall thruster code Hall2De is adapted to simulate a low temperature propulsive magnetic nozzle. Electron cyclotron resonance (ECR) thrusters developed at the Office National d’Etudes et de Recherches Aérospatiales (ONERA) in France and the University of Michigan are investigated. For each device, the simulated domain is defined from the nozzle throat to a location 10 thruster radii downstream. A numerical mesh is defined based on the measured magnetic field, and the governing fluid equations for singlycharge ions, electrons, and neutrals are solved. The boundary conditions for the throat of the magnetic nozzle are based on measured values of electron temperature and ion velocity. Multiple boundary conditions for the electron temperature in the downstream regions of the domain are considered. The simulated ion velocity and potential agrees well with experiment, while the simulated divergence efficiencies and thrusts are 25% lower and 45-55% higher than experiment. When a Neumann boundary condition for electron temperature is applied to the far-plume boundaries, the nozzle was seen to be purely isothermal. This suggests that the thermal conductivity along field lines in real devices is much lower than classical simulations predict, and anomalous resistivity may be needed to reproduce the temperature distribution observed in experiment. The larger beam divergence observed in our simulation may also be related to the lack of anomalous resistivity.

Jorns, Benjamin A.↗
NASA NTRS2019

Numerical investigations of background pressure effects and channel erosion in the SPT-140 Hall thruster for the Psyche mission

Four SPT-140 Hall thrusters are the main components of the electric propulsion system for the Psyche mission. We employ the 2-D (r-z) code Hall2De to conduct numerical simulations of the SPT-140 with the goal of estimating erosion of the acceleration channel rings for the Psyche mission profile. The simulations are informed by direct measurements of the plasma conditions in the acceleration channel that were obtained using laser-induced fluorescence. We validate our simulation results with additional plasma measurements, wear test erosion rates, and performance data as a function of background pressure. We then make use of a series of numerical simulations to compute the progressive erosion of the channel walls, using as input parameters a profile for the Psyche mission that includes operation at 4.5, 2.5 and 1.0 kW with duration of all segments increased by a 50% margin. We find that the channel walls are not completely eroded at the end of the mission (plus 50% margin). We also observe a progressive decrease in thrust as the channel walls erode that is consistent with the wear test and flight data, with a rate of ~4.5 mN/kh for the first 2,500 h and constant thrust after that. The reasons for these thrust trends are explained.

Lenguito, Giovanni↗
NASA NTRS2019

Mechanism behind the dependence of thrust on facility backpressure and implications on the operation of the SPT-140 onboard the Psyche mission

The Psyche mission will employ Stationary Plasma Thrusters (SPT-140) for primary propulsion in space and will be the first to use Hall thrusters beyond lunar orbit. One of the longest standing risks in the implementation of this EP technology on missions like Psyche is our limited understanding of the effects of ground test facilities on the performance and wear of the thruster.

Lenguito, Giovanni↗
NASA NTRS2019

Plasma simulations for the assessment of pole erosion in the Magnetically Shielded Miniature Hall Thruster (MaSMi)

Numerical simulations have been conducted to assess the erosion rates at the pole covers of the magnetically shielded miniature (MaSMi) Hall thruster. MaSMi is part of the Ascendant Sub-kW Transcelestial Electric Propulsion System (ASTRAEUS), whose design requirements demand a xenon throughput of 100 kg, thus enabling deep space missions using SmallSats. The results of our simulations show that the proposed thickness of the graphite pole covers will not be completely eroded at end of life. We find that the erosion rates are larger near the corners of the covers, which are directly exposed to the plasma in the acceleration channel.

Chaplin, Vernon H.↗
NASA NTRS2019

Completing the development of the 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS)

The Hall Effect Rocket with Magnetic Shielding (HERMeS) is a 12.5 kW Hall thruster co-developed by NASA Glenn Research Center and the Jet Propulsion Laboratory. HERMeS incorporates magnetic shielding to eliminate discharge channel erosion in order to reach its design lifetime of 50 kh at specific impulses up to 3000 s. The capabilities of the HERMeS thruster technology transferred to Aerojet Rocketdyne under the Advanced Electric Propulsion System (AEPS) program are described. HERMeS hardware testing is now focused at reducing risk and supporting the qualification of the AEPS thruster. These includes a series of progressively longer wear tests, plasma characterization and modeling supporting life qualification, magnetic field optimization, and environmental testing. Initial results from AEPS thruster testing show operation consistent with the HERMeS thrusters and steady-state operation has been achieved at 600 V, 12.5 kW.

Herman, Daniel↗
NASA NTRS2017

Work function reduction in lanthanum hexaboride hollow cathodes operated in gas discharges

Thermal characterization of lanthanum hexaboride (LaB6) hollow cathodes has revealed lower than expected electron emitter temperatures when the cathode reaches steady state. This phenomenon is observed at discharge currents ranging from 5 to 35 A and xenon mass ow rates of 5 to 25 sccm in cathodes with three di erent ori ce diameters. Thus, the accepted value of the work function for polycrystalline LaB6, 2.66 eV, does not describe well the emission characteristics of LaB6 hollow cathodes operating with internal gas discharges at steady state. The measured temperatures and a model of the hollow cathode emitter and xenon discharge were used to estimate the value of the work function in these experiments, yielding a value ranging from 2.1 - 2.44 eV. Measurements of the work function as a function of depth on a hollow cathode emitter using X-ray photoelectron spectroscopy and ion beam milling indicate that the work function in a thin layer near the emitting surface is lower than in the bulk material. We postulate that a lanthanum-rich emitting surface with a lower work function develops as a result of lanthanum recycling in the internal plasma.

Lopez Ortega, Alejandro↗
NASA NTRS2017

Inner Front Pole Cover Erosion in the 12.5 kW HERMeS Hall Thruster Over a Range of Operating Conditions

Erosion characteristics on the cover of the inner front pole in a 12.5 kW Hall thruster were measured over a wide range of operating conditions in tests of 6 to 14 hours duration using an accelerated test method and a very sensitive radioactive tracer-based erosion diagnostic. The operating points included the nominal 300 - 600 V conditions on a constant 20.8 A throttle curve, but included additional conditions at other currents spanning the throttling envelope and measurements at varying magnetic eld strength, facility pressure, and discharge voltage oscillation amplitude. The results show that the 300 V condition produces the highest wear rates on the 20.8 A throttle curve, but that rates actually increase with decreasing current. The wear rate was insensitive to discharge voltage ripple, but increased monotonically with magnetic eld strength, particularly near the inner radius of the pole cover. The inner region was also sensitive to facility pressure, showing lower rates at a higher pressure level. Separate experiments in which the energy distributions of ions generated by the hollow cathode were measured suggest that the cathode plume may be a source of energetic ions responsible for some of the erosion trends, in addition to ions originating in the thruster plume. The Hall thruster simulation code Hall2De is able to reproduce the erosion characteristics observed at 600 V, 20.8 A, but cannot currently match the rates measured at lower voltages and currents.

Lopez Ortega, Alejandro↗
NASA NTRS2017

A First-Principles Model Based on Saturation of the Electron Cyclotron Drift Instability for Electron Transport in Hydrodynamics Simulations of Hall Thruster Plasmas

We present a computational model based on the hypothesis that the higher-than-expected electron mobility across magnetic field lines observed in Hall thrusters is due to the growth and eventual saturation of the electron cyclotron drift instability. The key improvement of this model with respect to previous work is that it decouples the saturation of the instability from the correlation that exists between mobility and magnitude of the wave perturbations in linear theory, thereby enabling first-principles simulations. The model has been incorporated in the 2-D (r-z) multi-fluids code Hall2De and simulations of the H6 thruster at 300 V, 20 A are numerically stable and achieve a steady-state solution at a computational cost that is not significantly higher than previous Hall2De simulations. The computed location of the acceleration region is found to be within 10% of the length of the acceleration channel compared to that inferred by experiments. The simulations also capture well the plasma gradients along the channel centerline of this thruster. To further establish the validity of this model, we plan to conduct simulations of other thrusters and/or operating conditions.

Lopez Ortega, Alejandro↗
NASA NTRS2016

Hall2De Simulations with a First-Principles Electron Transport Model Based on the Electron Cyclotron Drift Instability

Several years of work have combined plasma measurements in a laboratory Hall thruster and r-z numerical simulations with Hall2De to isolate the spatial variation of the anomalous collision frequency needed in Ohm’s law to produce the observed thruster behavior. This numerical solution is used here to test the validity of a first-principles model of the anomalous transport in these devices before such model is implemented self-consistently in r-z fluid codes like Hall2De. The first-principles model employs quasi-linear theory and is based on the hypothesis that the Electron Cyclotron Drift Instability (ECDI) excites ion acoustic turbulence that, in turn, enhances the effective collision frequency in these devices. We find that a model of the ECDI that assumes Maxwellian velocity distributions for electrons and singly-charged, main-beam, cold ions (Ti=0.07 eV) is insufficient to explain the expected variation of the anomalous collision frequency both in the interior and exterior of the acceleration channel. When warm ions (~0.5-3 eV) are accounted for, the ECDI model in the channel interior appears more promising but fails by orders of magnitude in the near plume region due to the much higher Landau damping of the ion acoustic waves there. This implies that either (a) some process allows the ECDI instability to remain uninhibited by classical Landau damping or, (b) that a different instability (or instabilities) altogether, also insusceptible to Landau damping, is/are active in this region. A previous hypothesis, that convection of wave energy generated by the ECDI in the channel plays a significant role in the near plume, is not supported by the results of the simulations.

Mikellides, Ioannis G.↗
NASA NTRS2016

Mechanisms for Pole Piece Erosion in a 6-kW Magnetically-Shielded Hall Thruster

The mechanisms contributing to the erosion of the inner magnetic pole of a 6-kW magnetically shielded Hall thruster are experimentally and numerically investigated. Sputtering from ion bombardment is believed to be the cause of this wear, but the nature and source of the responsible ions remains unknown. Laser induced fluorescence and translating and surface-mounted probes are employed to characterize the flux and energy distribution of ions incident at the inner pole. It is found that the bulk population is comprised of ions originating from the cathode plume and the near field immediately adjacent to the inner pole. These ions have a low average kinetic energy, < 5 eV kinetic, but a high thermal spread in velocity, Ti = 10 eV. Calculations for the erosion at the pole from experimental measurements indicate that the bulk population only accounts for 10-25% of the observed erosion. It is postulated that the remainder of the erosion results from a separate population of ions with sufficiently high energy (> 150 V) to cause high sputtering but too low density to be detected experimentally by the plasma diagnostics in this region. The existence for this high energy population is shown to be a consequence of the high plasma potentials downstream of the thruster exit plane that are characteristic of a magnetically-shielded topography. A simulation validated by experimental measurements of the thruster provides further evidence that a high energy ion populations exists with sufficient density to explain the observed erosion.

Jorns, Benjamin A.↗